Plasma particulate filter

ABSTRACT

A method for reducing the particulate emissions containing carbon of diesel motors uses surface discharges to regenerate a filter. An appropriate wall flow filter is configured from alternately closed longitudinal channels. The electrodes are embedded in the filter material and are thus protected from erosion. Two electrodes are sufficient for selectively generating the surface discharges in the inlet channel of the wall flow filter as a result of a suitable geometric arrangement.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is based on and hereby claims priority to PCTApplication No. PCT/DE2003/002187 filed on Jul. 1, 2003 and GermanApplication No. 102 29 881.5, the contents of which are herebyincorporated by reference.

BACKGROUND OF THE INVENTION

The invention relates to a plasma particulate filter.

DE 100 57 862C1 discloses a plasma particulate filter and a method forreducing the levels of carbon-containing particulate emissions fromdiesel engines, in which the particulates contained in the exhaust gasare deposited on filter surfaces, the deposited particles being oxidizedin order to regenerate the filter, and the regeneration being effectedby non-thermal, electrical sliding surface discharges at the surfacescovered with particulates.

DE 100 57 862 C1 has described various geometries for operating anarrangement of this type which are based on the principle of what areknown as wall flow filters. These filters comprise parallel passageswith a quadrilateral cross section which are alternately closed on theoutlet side and the inlet side of the exhaust gas. This results in adivision into inlet passages for the particulate-laden exhaust gas andoutlet passages for the filtered exhaust gas. The particulates aredeposited on the inner walls of the passages that are open on the inletside and are oxidized there by oxygen and hydroxyl radicals which areproduced in the immediate vicinity of the wall by non-thermal slidingsurface discharge plasmas.

DE 100 57 862 C1 works on the basis that an electrode be arranged ateach of the edges of a filter passage in order to produce slidingsurface discharges. The electrodes required to produce plasma can eitherbe embedded in the filter material or applied to the filter material, insuch a way that in any event there is a layer with a high dielectricstrength between an electrode connected to high voltage and thecounterelectrode that is connected to ground. The embedding of theelectrodes described in that document, however, means that slidingsurface discharges can only be generated on both sides of the cellwalls, whereas the particulates are only deposited on one side. Thismeans that the specific energy consumption for the regeneration is twiceas high as is actually necessary.

On the other hand, electrodes which are exposed to the exhaust gas andare proposed in that document in combination with embedded electrodesfor the preferential operation of sliding surface discharges on one sideof the wall, on account of being in contact with the exhaust gas areexposed to erosion processes which may be boosted still further by gasdischarge processes. These erosion processes may not only have anadverse effect on the service life of the electrodes in particular, butalso, via the formation of metal oxides, on the service life of theceramic.

A further drawback is that the large number of electrodes—specificallyfour per inlet passage—significantly increases the size and weight ofthe plasma particulate filter compared to known filters.

The literature has disclosed geometries for the operation of dielectricbarrier discharges in ceramic honeycomb bodies (cf. for example EP 0 840838 B1), in which a cylindrical volume which includes a large number ofpassages could be excited by an internal high-voltage electrode and anexternal ground electrode. However, this means that it is not possibleto differentiate between inlet and outlet passages of a particulatefilter and also it is impossible to produce targeted sliding surfacedischarges. Moreover, the long sparking distance between the electrodesmeans that a high voltage amplitude of 20 kV is required, which can leadto problems in the motor vehicle.

SUMMARY OF THE INVENTION

Working on the basis of the latter related art, it is one possibleobject to provide a plasma particulate filter in which a suitablegeometry avoids the drawbacks listed above.

The inventor proposes a wall flow filter comprising elongate passages ofany desired cross section which are closed off on alternate sides, theparticulate-covered walls of which wall flow filter are regenerated bysliding surface discharges. On account of the arrangement of theelectrodes embedded in the filter material and thereby protected againstcorrosion, the sliding surface discharges now preferentially burn on theparticulate-covered inlet side of the filter. The geometry indicatedwith two-line symmetry advantageously requires only two electrodes perinlet passage to produce the sliding surface discharges.

The wall flow filter has elongate passages with a quadrilateral crosssection arranged in matrix form. The passages are closed off onalternate sides along a row or a column, so that inlet passages andoutlet passages alternate.

The electrode arrangement may ensure that the distribution of theelectric field in the individual cells of the plasma particulate filterallows non-thermal sliding surface discharges to be struck in individualcells. The dielectric properties of the wall material of the ceramicparticulate filter are utilized to concentrate the field in cavitiesbetween the electrodes. Surprisingly, a reduction in the number ofelectrodes per inlet passage from four to two does not, for example,result in a deterioration of the electric field distribution with regardto the generation of sliding surface discharges. For this to be thecase, it is important that the electrodes be arranged at diagonallyopposite edges of the quadrilateral passage cross section, and it isnecessary for inlet passages which are adjacent via their edges whichare not provided with electrodes to be connected so as to have the samepolarity.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects and advantages of the present invention willbecome more apparent and more readily appreciated from the followingdescription of the preferred embodiments, taken in conjunction with theaccompanying drawings of which:

FIG. 1 and FIG. 3 show cross sections through plasma filter elementswith inlet passages and outlet passages and associated electrodes,

FIG. 2 and FIG. 4 show calculated field strength distributions in thearrangements shown in FIGS. 1 and 3, and

FIG. 5 shows cross sections through an inlet passage with two-linesymmetry and its variation.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Reference will now be made in detail to the preferred embodiments of thepresent invention, examples of which are illustrated in the accompanyingdrawings, wherein like reference numerals refer to like elementsthroughout.

The figures are in part described jointly below. In particular inconnection with FIG. 1, reference is made in detail to patent DE 100 57862 C1.

The latter patent protects a method and associated arrangements forlowering the levels of carbon-containing particulate emissions fromdiesel engines in which sliding surface discharges are used. FIGS. 1 to5 and 7 to 12, which are described in detail in DE 100 57 862 C1,illustrate wall flow filters made from ceramic material composed ofelongate passages which are closed off on alternate sides and have aspecial quadrilateral cross section with electrodes fitted at each ofthe corners.

FIG. 1 shows a cross section through an electrode arrangement of thistype in a plasma filter element of known design with four electrodes perpassage embedded in filter material.

In detail, an inlet passage is denoted by 10 and an outlet passage isdenoted by 20. Inlet passage 10 and outlet passage 20 are separated byporous walls 30 made from specific ceramic material. Electrodes arefitted in the walls 30 at each of the corners of the passages 10, theseelectrodes, arranged in pairs next to one another, serving as ahigh-voltage electrode 41 and a grounded electrode 42. To ensuresufficient dielectric strength, the electrodes 41 and 42, made fromelectrically conductive material, are each surrounded by an electricallyinsulating barrier layer 31, which to enable it to withstand highvoltages has a low porosity compared to the filter material of the walls30.

FIG. 2 shows the distribution of the electric field strength, which isof importance to the formation of sliding surface discharges, for avoltage of 10 kV applied to the high-voltage electrodes in the case of asquare passage cross section of 2×2 mm² in cross section through thearrangement shown in FIG. 1. 50 denotes calculated field minima in thearrangement shown in FIG. 1. On account of the quadrupole-likearrangement of the electrodes, these minima are in each case located onthe axes of symmetry of both the inlet passages and the outlet passages.Regions with an elevated electric field strength 51, in which electricgas discharges are preferentially struck, are located in the vicinity ofthe passage walls of both the inlet passages and the outlet passages.

Overall, it can be seen from FIG. 2 that on account of the symmetry inthe outlet passages 20, the same electric field distribution as in theinlet passages 10 results. However, for particulate oxidation in thewall flow filter, the regions of elevated electric field strength areactually only required in the inlet passages.

FIG. 3 shows an electrode arrangement for the selective production ofgas discharges in the inlet passages, in cross section. The maindifference with respect to FIG. 1 is the diamond-shaped arrangement ofthe inlet passages 10 and the outlet passages 20, which results from thestructure shown in FIG. 1 being rotated through 45°. A furtherdifference with respect to the related art is that electrodes 40, whichare in this case designed in pairs as a high-voltage electrode 41 and aground electrode 42, are in each case present at opposite corners of thediamond on the vertical at the inlet passages, which are now ofdiamond-shaped design. In this case too, for a porous filter material abarrier layer 31 is provided once again.

FIG. 4 shows the advantageous distribution of the electric field in thearrangement shown in FIG. 3, which allows the preferential ignition ofgas discharges within the inlet passages. It is clear from thiscalculated illustration that compared to FIG. 2 the inlet passages 10have an elevated electric field strength which is sufficient to ignitegas discharges over virtually the entire cross section, whereas in theoutlet passages 20 the ignition of gas discharges is only likely in thevicinity of the electrodes, on account of slightly elevated electricfields. Otherwise, field minima 50 are once again present in accordancewith FIG. 2.

Preferred attachment points for gas discharges in the inlet passages 10are firstly in the vicinity of the electrodes on account of the elevatedelectric field strength being particularly pronounced there. However,since electric charge carriers are stored during operation of the gasdischarge, and therefore the electric fields are reduced there, thepreferred points of attachment for the gas discharges gradually slidealong the walls of the inlet passages 10 toward the center region untilthe walls are covered with surface charges to such an extent that it isno longer possible to ignite any further gas discharges.

The latter process is associated with the formation of sliding surfacedischarges. Although the initial field distribution allows slidingsurface and volume discharges equally, in this way, a not insignificantpart of the electrical energy is converted into sliding surfacedischarges. At the same time, the operation of gas discharges in theoutlet passages is substantially suppressed. This confirms that thearrangement shown in FIG. 3 gives an improved result, compared to FIG.1, which corresponds to the related art, for the implementation of aplasma particulate filter with the use of sliding surface discharges foroxidation of the particulates.

The arrangement shown in FIG. 3, compared to FIG. 1, not only results inan electric field distribution which is advantageous for the efficientutilization of the electrical energy, but also results in a reduction ofthe materials and costs outlay as a result of a reduced number ofelectrodes per unit filter volume and area and, at the same time, areduced electrical capacitance, which has the effect of reducing costson account of simplification of the design of high-voltage grid partsfor electrical excitation of the plasma particulate filter. In thiscontext, it is important for the electrodes to be arranged at diagonallyopposite edges of the quadrilateral passage cross section; inletpassages which are adjacent via their edges that are not provided withelectrodes must necessarily be connected so as to have the samepolarity.

FIG. 5 shows, as an excerpt from FIG. 3, on the left-hand side thediamond-shaped cross section of an individual inlet passage withelectrode 41, counterelectrode 42 and two axes 60 and 60′ which define atwo-line symmetry. These elements are of importance for the ability ofthe filter to function, the electrodes 41 and 42 being connected by theaxis 60 as one line of symmetry.

It will be clear that the concept described can also be transferred toother passage cross sections. Working on the basis of the overallgeometry shown in FIG. 3 and the specific symmetry presented in FIG. 5,the electrodes 41 and 42 and the connecting axis 60 between theelectrodes 41 and 42, as a first line of symmetry, are held in place andthe passage cross section is deformed symmetrically with respect to thisaxis. When the second line of symmetry is taken into account, theresult, for example, is a star shape in the right-hand part of FIG. 5,in which the wall surface area which is active in the deposition ofparticulates is increased in the inlet passage compared to FIG. 3.

If the geometry in accordance with FIG. 5 is taken into account, theoutlet passages are deformed in a correspondingly complementary way, sothat the cross section is once again completely covered with inlet andoutlet passages. In principle, any conversion of a quadrilateral into ann×quadrilateral with n≧2 is possible.

The invention has been described in detail with particular reference topreferred embodiments thereof and examples, but it will be understoodthat variations and modifications can be effected within the spirit andscope of the invention covered by the claims which may include thephrase “at least one of A, B and C” or a similar phrase as analternative expression that means one or more of A, B and C may be used,contrary to the holding in Superguide v. DIRECTV, 69 USPQ2d 1865 (Fed.Cir. 2004).

1-8. (canceled)
 9. A plasma particulate filter comprising: a mesh ofceramic filter material having a first set of walls extending in a firstdirection and a second set of walls extending in a second direction, thefirst set of walls intersecting the second set of walls to formsubstantially parallel inlet and outlet passages, the inlet and outletpassages being closed at alternate ends of the mesh, each passage havinga cross section with four corners defined by intersections of the firstand second set of walls; and precisely two active electrodes providedfor each inlet passage to oxidize particulates deposited within theinlet passage on the filter material within the inlet passage, theelectrodes being formed at intersections of the first and second sets ofwalls such that for each inlet passage, an electrode is provided each oftwo diagonally opposing corners, the electrodes having oppositepolarities, each electrode serving two inlet passages.
 10. The plasmaparticulate filter as claimed in claim 9, wherein the electrodes areembedded in the filter material.
 11. The plasma particulate filter asclaimed in claim 9, wherein the electrodes are embedded in anelectrically insulating barrier material of low porosity.
 12. The plasmaparticulate filter as claimed in claim 9, wherein the inlet passageseach have a cross section with two-line symmetry and with n×4 corners,where n≧2, and the n×4 corners are obtained by deformation of aquadrilateral cross section.
 13. A plasma particulate filter based on awall flow filter, comprising: elongated inlet and outlet passages whichare closed on alternate sides and which are made from ceramic filtermaterial such that particulates are deposited on surfaces of the filtermaterial within the inlet passages, the passages each having a crosssection with two-line symmetry; and precisely two electrodes ofdifferent polarity, lying on one of the lines of symmetry, per inletpassage, the electrodes regenerating the filter by oxidizing theparticulates through dielectric barrier sliding surface discharges. 14.The plasma particulate filter as claimed in claim 13, wherein theelectrodes are embedded in the filter material to protect the electrodesagainst erosion.
 15. The plasma particulate filter as claimed in claim13, wherein the electrodes are embedded in an electrically insulatingbarrier material of low porosity.
 16. The plasma particulate filter asclaimed in claim 13, wherein the electrodes are positioned to generatesliding surface discharges that selectively burn particulates on theinlet passages.
 17. The plasma particulate filter as claimed in claim13, wherein the cross section of the passages with two-line symmetry hasa quadrilateral geometry, the two electrodes being arranged at oppositecorners of the quadrilateral geometry.
 18. The plasma particulate filteras claimed in claim 17, wherein the quadrilateral geometry is avertically oriented diamond.
 19. The plasma particulate filter asclaimed in claim 18, wherein the inlet passages have adjacentdiamond-shaped cross sections, electrodes are arranged at diagonallyopposite corners of a plurality of diamond-shaped cross sections, andfor adjacent inlet passages, the electrodes at the corners are connectedso as to have the same polarity.
 20. The plasma particulate filter asclaimed in claim 13, wherein the inlet passages each have a crosssection with two-line symmetry and with n×4 corners, where n≧2, and then×4 corners are obtained by deformation of a quadrilateral cross sectionwhile maintaining the electrodes on one of the lines of symmetry. 21.The plasma particulate filter as claimed in claim 14, wherein theelectrodes are embedded in an electrically insulating barrier materialof low porosity.
 22. The plasma particulate filter as claimed in claim21, wherein the electrodes are positioned to generate sliding surfacedischarges that selectively burn particulates on the inlet passages. 23.The plasma particulate filter as claimed in claim 22, wherein the crosssection of the passages with two-line symmetry has a quadrilateralgeometry, the two electrodes being arranged at opposite corners of thequadrilateral geometry.
 24. The plasma particulate filter as claimed inclaim 23, wherein the quadrilateral geometry is a vertically orienteddiamond.
 25. The plasma particulate filter as claimed in claim 24,wherein the inlet passages have adjacent diamond-shaped cross sections,electrodes are arranged at diagonally opposite corners of a plurality ofdiamond-shaped cross sections, and for adjacent inlet passages, theelectrodes at the corners are connected so as to have the same polarity.26. The plasma particulate filter as claimed in claim 25, wherein theinlet passages each have a cross section with two-line symmetry and withn×4 corners, where n≧2, and the n×4 corners are obtained by deformationof a quadrilateral cross section while maintaining the electrodes on oneof the lines of symmetry.